The invention relates to optical devices and methods for optical threshold determination and for performing comparison functions, as are used in signal regeneration, pattern recognition and optical computing applications.
FIG. 1 of the accompanying drawings shows an optical device as proposed by Gray et al [1]. The proposed device is an amplifier made of an active gain medium such as a rod used for a solid state laser. In operation, one signal I1 is injected straight through the gain medium and another signal I2 is injected in a zigzag path defined by total internal reflections from the sides of the gain medium. The zigzag path is longer than the straight path and thus has a higher net gain. Its output is thus more sensitive to variations in the gain of the gain medium than the output from the straight path. This allows a signal injected in the lower gain straight path to control the gain of a signal travelling in the zigzag path by cross saturation of the gain, i.e. cross gain modulation (XGM). Light input into the amplifier along the straight path thus represents a control input, whereas light input along the zigzag path represents a signal input to be modulated.
FIG. 2 of the accompanying drawings shows another optical device proposed by Gray et al [1] (see FIG. 10(a) of that reference) which is an optical differential comparator based on two of the above-described amplifiers connected in a feedback configuration. A portion of the signal output from a first amplifier 1 is fed back to the control input of a second amplifier 2 by means of a partially reflecting mirror 3 and further routing components 4 which may be a sequence of waveguides or mirrors. The feedback action results in operation analogous to that of an electrical differential amplifier or comparator, as illustrated in FIG. 10(b) of reference [1 ]. Threshold and logic functions are also discussed in reference [1 ].
FIG. 3 of the accompanying drawings shows a further prior art optical device according to Parolari et al [2]. This device may be considered to be a development of the free-space device of FIG. 2 proposed in reference [1]. The device comprises two amplifier elements 120 and 130 analogous to the corresponding elements 1 and 2 of FIG. 2. These are implemented as semiconductor optical amplifiers (SOA""s). A portion of the output signal from the first SOA 120 is fed back to the input of the second SOA 130. Similarly, a portion of the output signal from the second SOA 130 is fed back to the input of the first SOA 120. Input and output side three-way couplers 123, 121, 133 and 131 (C11, C21, C12 and C22) are provided for this purpose, as illustrated.
In the prior art free-space device of FIG. 2, the two beams are separated spatially and thus do not interact. By moving to the waveguide implementation of FIG. 3, spatial separation is lost. To overcome this problem, spectral separation is provided by using two different wavelength xcex1, and xcex2 and inserting first and second optical filters 129 and 139 after their respective SOA""s in order to separate the interacting signals. The first optical filter 129 is transmissive at the wavelength xcex1 of the first SOA 120 but absorptive at the wavelength xcex2 of the second SOA 130. This prevents the feedback signal Pfeedback(xcex2) being transmitted to the coupler 121 and on to the device output together with the output signal Pout(xcex1). Similarly, the second optical filter 139 is transmissive at the wavelength xcex2 of the second SOA 130 but absorptive at the wavelength xcex1 of the first SOA 120. The filters 129 and 139 may be bandpass or cut-off filters. Bragg reflectors could be used, for example.
In addition, optical isolators 124 and 134 are inserted before both SOA""s. This helps suppress problems arising from the use of SOA""s as the non-linear active gain medium. These SOA problems may arise from: (a) the high small signal gain for unit length; (b) parasitic oscillations which may occur due to reflections in the circuit decrease the gain available for compression; and (c) the total device efficiency. Coupling ratios are chosen from a trade off between the feedback and output powers.
In summary, the device of FIG. 3 may be considered to be a waveguide development of the free-space device of FIG. 2 in which two wavelengths xcex1 and xcex2 are used instead of one, in conjunction with optical filters 129 and 139, and the isolators 124 and 134.
The maximum bit rates that can be handled by the prior art devices of reference [1] and reference [2] would be limited by the carrier lifetime in the active gain medium. In addition, the inventors have also realized that the maximum bit rates are also limited by the propagation time associated with the feedback paths. The propagation time is defined by the device topology and would limit the ultimate maximum speed of any such device, if the active gain medium response time is very short.
The XGM effect is also discussed by Fatehi et al [3] in relation to Erbium doped fiber amplifiers (EDFA""s). Moreover, it has been proposed to exploit the related effect of cross phase modulation (XPM) in various interferometer devices [4][5][6].
According to a first aspect of the invention there is provided an optical device comprising:
(a) a first active medium having a first propagation path for traversal of a first optical signal in a first forward direction;
(b) a second active medium having a second propagation path for traversal of a second optical signal in a second forward direction; and
(c) a feedback path connecting the first and second active media so as to route at least a portion of the first and second optical signals, after traversing the first and second active media, to the second and first active media as respective second and first optical control signals which travel along the second and first propagation paths in second and first reverse directions that are opposite to the second and first forward directions respectively.
According to a second aspect of the invention there is provided a method of modulating an optical signal, comprising:
(a) providing first and second active media;
(b) supplying first and second optical signals to traverse the first and second active media in first and second forward directions; and
(c) routing at least a portion of the first and second optical signals, after traversing the first and second active media, to the second and first active media as second and first optical control signals respectively, wherein the first and second optical control signals are supplied through the first and second active media in first and second reverse directions opposed to the first and second forward directions so that the first and second optical control signals vary the modulation experienced by the first and second optical signals.
The device and method of the first and second aspects of the invention thus fundamentally differ from those of references [1] and [2] in that the optical control signal is supplied to the active medium in the opposite direction to the optical signal to be modulated, instead of in the same direction.
Whether the optical control signal is supplied in a co-propagating configuration, as in the prior art, or in the counter-propagating configuration according to the invention has little influence on the XGM effect. However, counter-propagation of the optical control signal means that separation of the optical signal and optical control signal can be obtained by the direction of propagation alone. Consequently, the optical signal and optical control signal can be the same wavelength if desired. Moreover, there is no requirement to separate out the optical control signal from the optical signal at the output, since it does not appear at the output, but rather at the input of the device, where it can be filtered out if desired by a conventional isolator, for example to suppress parasitic oscillations.
With the counter-propagating configuration, spatial filtering by use of different optical paths as in reference [1] is also not necessary. (This type of filtering would in any case be difficult for typical optical fiber or planar waveguide based devices). For the same reason, wavelength filtering is not necessary even when the optical control signal and optical signal share the same optical path. In other words, wavelength selective filters such as band pass or band reject filters are not needed.
The counter-propagating feedback configuration allows much shorter feedback path lengths to be achieved in comparison to what is possible with a co-propagating configuration. Consequently, much higher bit rates can be achieved, since the feedback time can be made commensurately shorter. It is estimated that the feedback path length can be made around 10 times shorter with the proposed counter-propagating configuration, than with a co-propagating configuration.
A device according to the first aspect of the invention may be implemented with optical fiber connections and fiber couplers. Alternatively, the device may be implemented as an integrated planar waveguide device using planar waveguide connection and Y-couplers. Free-space implementation could also be achieved, but this is not envisaged to be particularly interesting for most practical applications.
In an optical fiber implementation, the device has fewer components and lower losses than a comparable device having a co-propagating configuration. Moreover, the device will have a lower switching power, switching power being defined as the minimum input power needed to obtain a given extinction ratio.
In a planar waveguide implementation, the device will be much simpler to fabricate and bending losses can be significantly reduced, compared to a co-propagating device. In a co-propagating device, it is inevitable that the two feedback paths will need to cross, and this will add complexity to the fabrication in planar waveguide technology. By contrast, with the proposed counter-propagating feedback architecture, there is no such crossing to implement, the structure being inherently compatible with planar technology.
The proposed novel devices and methods based on counter-propagation of the optical control signals thus offer several major advantages which cannot be achieved with comparable co-propagating devices.
The device of the first aspect of the invention can operate as a threshold circuit for determining a threshold according to the method of the second aspect of the invention. Threshold determination has many applications.
For example, signal regeneration can be obtained by reshaping an optical signal which enters an optical decision circuit. All signal levels below a certain decision threshold are transformed into a constant low level, and all signal levels above the decision threshold are transformed into a constant high level.
Another application is in an all-optical pattern recognition device which needs a threshold to distinguish between auto-correlation values (when a target is recognized) and cross-correlation values (non-target sequences).
Moreover, comparison functions can also be employed in optical computing applications.
In one embodiment of the first aspect of the invention, the first and second active media are arranged so that their optical signal outputs face in the same direction. In other words, the first and second active media are arranged with their first and second forward directions aligned. The feedback path comprises a bend which is arcuate and passes through approximately 180 degrees.
In another embodiment of the first aspect of the invention, the first and second active media are arranged with their first and second forward directions opposed. The first and second output ports face towards each other. The feedback path is substantially straight, that is free of curvature liable to produce significant bending losses.
The active media of the first and second aspects of the invention may be gain media or lossy media so that the optical signals are amplified to varying degrees or attenuated to varying degrees depending on the optical control signals.